Satellite Positioning System receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites such as GPS (or NAVSTAR) satellites. For example, the U.S. Global Positioning System (GPS) Orbital Constellation consists of 24 satellites which orbit the earth in 12 hour orbits. The satellites are arranged in six orbital planes each containing four satellites. The orbital planes are spaced 60 degrees apart from each other and are inclined approximately fifty-five degrees with respect to the equatorial plane. This constellation provides a user with approximately five to eight satellites visible from any point on earth. These satellites transmit, as part of their message, both satellite positioning data, so-called "ephemeris" data, as well as clock timing data. In addition, the satellites transmit time-of-week (TOW) information associated with the satellite signal, which allows the receiver to unambiguously determine local time. The process of searching for and acquiring GPS signals, reading the ephemeris and other data for a multiplicity of satellites and then computing the location of the receiver (and accurate time-of day) from this data is time consuming, often requiring several minutes of time. In many applications, this lengthy processing time introduces unacceptable delays, and furthermore, greatly limits battery life in portable applications utilizing miniaturized circuitry.
In addition, in many situations there may be blockage of the satellite signals. In these cases, the received signal level from the GPS satellites can be too low to demodulate and derive the satellite data signals without error. Such situations may arise in personal tracking and other highly mobile applications. In these situations, it is usually possible for a receiver to continue to acquire and track the GPS signals. However, performing location and unambiguous time measurement without such data requires alternative methods.
It is therefore desirable to provide a system for providing time information to a GPS receiver without requiring the receiver to derive timing information from GPS signals received from GPS satellites or from an internally generated clock. It is further desirable to provide a system which derives timing information for GPS applications from timing signals contained in communications transmissions received by the receiver.